folder space on disk

Does the ubuntu one folder on my computer take up duplicate space for the contents? This doesn't matter on my main computer but my netbook only has an 8GB home folder so I can't afford any duplication.
I am also worried that if I upgrade to 20GB storage as a back-up for the main computer that will appear in the folder on the netbook and swamp it.
Can someone explain these things simply? I have been searching without success.